The present invention relates to porous sheets comprising thermoplastic polymer, more particularly porous sheets having specific porous structures on a cross section and roughened network structures on at least one surface of the sheets.
Non-woven fabrics comprising a collection of fibers have been known. Fibers forming non-woven fabrics are held together by adhesion or fusion at intersectional portions. Although part of fibers have sectional shapes some what deformed by heating, substantial part of fibers in the surface and inside of non-woven fabrics are in collected arrangement retaining their original shapes. Non-woven fabrics made of entangled fibers formed by needle punch entanglement or water jet entanglement instead of bonding fibers by adhesive or heat have been also known. These non-woven fabrics have been used in wide applications of clothing or non-clothing fields due to good flexibility, good air permeability and mellow appearance. However, known non-woven fabrics are unsuitable in some applications, for example, due to fluff formation during their use and requirement for air permeability smaller than usual.
As air-permeable materials, porous films have been known, which are manufactured by a method where thermoplastic polymer incorporated with a void-forming agent is stretched, a method where a void-forming agent is foamed during film-forming process, a method where a void-forming agent is extracted after film formation to produce porous structure, a method where a solution of a thermoplastic polymer and a void-forming agent is extruded into a coagulation bath to coagulate thermoplastic polymer solution thereby to produce porous structure, etc. Although the fine structures of porous films may be easily controlled in these methods, the selection of suitable void-forming agent is required in addition to relatively complicated production steps. In addition, thermoplastic polymer incorporated with a void-forming agent in an amount sufficient for obtaining intended air permeability is in some case difficult to be made into films depending on the addition amount, or fails to give films having practically high strength even when it can be made into films.
As described above, known non-woven fabrics and porous films do not meet the properties required in some application fields.
For example, in specific application as the substrates of first-aid sticking plasters, conventional substrates of low friction and high abrasion resistance are mainly made of films having poor air permeability. To make the film substrates air-permeable, holes should be formed in films or porous films should be used. However, films are poor in tactile properties and texture.
To improve the tactile properties and air permeability of films, for example, Japanese Utility Model No. 1917955 proposes to use sheets made of polyurethane elastic fibers as the substrates for first-aid sticking plasters. However, since the substrates are made of fiber sheet, fluffing occurs due to surface rubbing during their use.
Although the fluff formation may be reduced by increasing fiber size and fusing fibers at their intersections, this in turn makes texture excessively roughened. Alternatively, it is proposed to heat-treat the surface of fiber sheet, this making the production process complicated due to additional step of heat calendar treatment.